The balance of nature is a very elaborate and delicate system of check and countercheck. I

t is continually being altered as climates change, as new organisms evolve, as animals or plants spread to new areas. But the alterations have in the past, for the most part, been slow, whereas with the arrival of civilized man, their speed has been multiplied much: from the evolutionary time-scale, where change is measured by periods of ten or a hundred thousand years, they have been transferred to the human time-scale in which centuries and even decades count.

Everywhere man is altering the balance of nature. He is facilitating the spread of plants and animals into new regions, sometimes deliberately, sometimes unconsciously. He is covering huge areas with new kinds of plants, or with houses, factories, slag heaps and other products of his civilization. He gets rid of some species on a large scale, but favors the multiplication of others. In brief, he has done more in five thousand years to alter the biological aspect of the planet than has nature in five million years.

Many of these changes which he has brought about have had unforeseen consequences. Who would have thought that the throwing away of a piece of Canadian waterweed would have caused half the waterways of Britain to be blocked for a decade？ Or that provision of pot cacti for lonely settlers' wives would have led to Eastern Australia being covered with forests of prickly pears？

Who would have predicted that the cutting down of forests on the Adriatic coast, or in parts of Central Africa, could have reduced the land to a semi-desert with the very soil washed away from the bare rock？ Who would have thought that improved communications would have changed history by the spreading of disease-sleeping sickness into East Africa, measles into Ocean, AIDS around the whole world？

These are spectacular examples, but examples on a smaller scale are everywhere to be found. We made a nature sanctuary for rare birds, providing absolute security for all species ; and we may find that some common and hardy kind of birds multiplies beyond measure and drive away the rare kinds in which we are particularly interested. We see, owing to some little change brought about by civilization, the starting spread over the English countryside in hordes. We improve the yielding capacities of our cattle; and find that how they exhaust the pastures which were sufficient for less demanding stock. We gaily set about killing the carnivores that disturb our domestic animals, the hawks that eat our fowls and game-birds; and find that in so doing we are also removing the brake that restrains the multiplication of mice and other little rodents that gnaw away the farmers' profits.

The author's concern with the balance of nature, as evidenced by this passage, is best defined as______.

A．intense and objective

B．conscientious but humorless

C．sincere and subjective

D．deep but dry